The invention relates to a method for the ambulatory detection and diagnosis of the sleep apnea syndrome, in which the physiological parameters of heart rate, loud breathing sounds and snoring sounds of the patient are detected and corresponding signals are generated. Sets of these that are detected in brief time intervals are stored in coded form in a portable apparatus, transferred to a computer, and analyzed by the latter, and especially variations of the individual signals in time and correlations between the various parameters are taken into account. The invention furthermore relates to an apparatus necessary for the practice of this method.
The symptom of sleep apnea is characterized by the combination of a respiratory failure, a considerable hypoxemia (i.e., a reduction of the oxygen content in the arterial blood) and heart rhythm disturbances. After an episode of apnea, a severe gasping for air and often also a terrifying awakening occur as a rule (see, for example, M. J. Tobin, M. A. Cohn, M. A. Sackner: Breathing abnormalities during sleep, Arch. Intern. Med. 1983; No. 143, pp 1221 to 1228).
The epidemiological significance of sleep apnea is increasingly being recognized; it is known that long and frequent phases of apnea in sleep often accompany cardiovascular and cardiopulmonary diseases as well as extensive psychophysical changes. Effects of sleep apnea are an excessively increased tendency to fall asleep during the day (in this case sleep apnea is statistically identified as the most frequent cause) and the occurrence of difficulty in falling asleep and sleeping through the night (in this case sleep apnea is described as the fifth most frequent cause (R. M. Coleman, H. P. Roffwarg, S. J. Kennedy: Sleep-wake disorders based on polysomnographic diagnosis. A national cooperative study. JAMA 1982; No. 247, pp. 997 to 1003)).
For some time the study of sleep-wake disorders has been performed in sleep laboratories in special clinics, in which a diagnosis can be performed by polysomnographic evaluation during sleep. These studies are time-consuming and costly; on account of the many parameters that have to be recorded they can be performed only by confining the patient. In addition to the high costs, such confinement has the disadvantage that the patient's sleep is disturbed by strange surroundings, which diminishes the value of such studies. Consequently there have been a number of efforts to eliminate the disadvantages of diagnosis with confinement and to cope with the large number of patients caused by the high prevalence.
One possibility is to recognize and diagnose sleep apnea syndrome in an ambulatory manner with the aid of portable detection and recording apparatus, avoiding confinement in sleep laboratories.
U.S. Pat. No. 4,982,738 describes a method and an apparatus (NLMS-non-laboratory monitoring system) for the ambulatory detection and diagnosis of the sleep apnea syndrome corresponding to the kind referred to above. In this method heart potentials (EKG) are measured relative to a third electrode with two electrodes to be applied to the upper body of a patient and fed to a peak level detector. The heart rate is determined by the time intervals between the peak values. The breathing and snoring sounds are picked up by an electret microphone to be applied to the patient's larynx and fed to two different threshold detectors, one threshold detector being sensitive over the entire frequency range (about 100 Hz to 15 Khz) and the other being sensitive only in the lower frequency range (about 100 Hz to 800 Hz typical for snoring) on account of the damping of the high frequencies of the signals with a filter. The threshold detectors respond when the applied signal exceeds the preset thresholds. The thresholds are adjusted so that loud breathing sounds are detected by the first-named threshold detector, and snoring is detected with the second detector. These physiological parameters, namely heart rate and presence or absence of breathing and snoring are measured together at time intervals of one second, and stored in binary code for each time interval in a RAM situated in the apparatus. The data stored during a period of sleep are transferred from the mobile apparatus to a computer and analyzed on the computer for a sleep apnea syndrome. Information as to the presence of an apnea can be derived from the changes in time of the heart rate and the breathing and snoring sounds and the correlations between these parameters. The seriousness of an apnea depends on the frequency of its occurrences, their severity and their duration. The method using the described apparatus has the disadvantage that it permits only detection of occurrence and duration, which leads to high sensitivity but not sufficient specificity of diagnosis. Information on the body position of the patient, which is important to the diagnosis, say, to distinguish between walking and laying down or dependence of obstruction on position in bed was not available. In the case of disease conditions, such as polyneuropathy, arteriosclerosis or diabetes, the heart rate does not change greatly as it does in apnea attacks, but remains virtually constant. In the presence of such conditions apnea is difficult to diagnose, because differential diagnosis using oxygen saturation for detection of apneas was not possible. In many cases, therefore, when the apparatus of the state of the art is used, it must be followed by confinement in a sleep laboratory.